The long range goal of this research is to gain insight into mechanisms underlying the modulation of synaptic transmission. The relatively simple nervous system of Aplysia has advantages that have made it particularly useful for these studies. The specific aim of this grant is to explore the hypothesis that neuropeptides which are present in motor neurons are released during stimulation to modulate the effectiveness of conventional excitatory transmitters released from the same neurons. This will be investigated in a model system comprised of identifiable motor neurons which innervate feeding muscles and synthesize and contain characterized neuropeptides in addition to conventional excitatory transmitters such as acetylcholine. The neuropeptides have been shown to be potent modulators of neuromuscular transmission at feeding muscles. Different motor neurons contain different modulatory neuropeptides so it will be possible to determine if the peptides in a particular neuron "tailor" the plastic properties of neuromuscular transmission to individual muscles. There is a possibility that understanding fundamental neural processes such as modulation of synaptic transmission in Aplysia could have relevance to disorders of movement as well as more general disorders of affect and cognition.